Mobile IPv4 as a specific network layer protocol for mobile communications and Mobile IPv6 as another network layer protocol for mobile communications are the protocols solving the connectivity problem when IPv4 nodes roam in an IPv4 network or when IPv6 nodes roam in an IPv6 network. In other words, either mobile IPv4 or mobile IPv6 is used as routing support to permit IP nodes (hosts and routers) using IPv4 or IPv6 to seamlessly “roam” among IP subnetworks and media types. The Mobile IP method supports transparency above the IP layer, including the maintenance of active TCP connections and UDP port bindings.
During a transition period from IPv4 to IPv6, IPv4 and IPv6 networks are coexistent and a new upgraded IPv6 node may still want to communicate with an IPv4 node or access services inside an IPv4 network.
In case a node CN in a network using a specific protocol (e.g. an IPv4 node in a network using IPv4) wants to communicate with a node MN supporting the specific and another protocol and located in a network using the another protocol (e.g. an IPv4/IPv6 dual-stack mobile node in a network using Mobile IPv6), the node CN sends a packet to the node MN which passes through a router of a home network of the node MN supporting the specific and the another protocol. If the node MN is not in its home network, the router answers every packet received from CN and destined to MN with an error message, e.g. an ICMPv4 (Internet Control Message Protocol version 4) error message according to the specific protocol supported by the CN. In other words, the connection between the MN and the CN is broken because there is no IPv4 environment while the dual stack MN is roaming out of its dual stack home network into an IPv6 only network. Thus, known transition schemes can enable a communication between an IPv4 and an IPv6 node, but it is not possible to solve the roaming problem of a mobile node.
For example, when an IPv4 node tries to communicate with a mobile IPv6 terminal which also supports IPv4, a packet sent from the IPv4 node passes through a dual stack router of the home network of the mobile IPv6 terminal. In case the mobile IPv6 terminal is away from its home network, the router answers the packet with an ICMP (Internet Control Message Protocol) error ‘host unreachable’. This means that the mobile IPv6 terminal is not reachable for the IPv4 node when the mobile node roams outside its home network.
Assuming a dual stack mobile node MN supporting the protocol of its home network such as IPv6 as well as a protocol of a correspondent node CN located in another network using e.g. IPv4, the mobile node MN is able to reach the correspondent node CN even if the mobile node MN is not located in its home network but roams in a visited network using IPv6. However, the correspondent node CN having a conventional IPv4 connection cannot reach the roaming mobile node MN. Rather, in an IPv4 connection to an IPv6 network, the error ‘host unreachable’ is generated in case a host does not attach to its original network.